There are three basic steps for constructing plots with ggplot2:

1. Supply a data set you want to plot to ggplot().
2. Define an aesthetic mapping with aes().
   This describes how variables of your data are mapped to visual properties, e.g. variable "age" is plotted on the x-axis and "income" on the y-axis.
3. Add layers of geoms (geometrical objects) that describe which graphical primitives to use (e.g. points in a scatter plot or bars in a bar plot).

Additionally, you can further change the appearance of your plot by:

• altering the scales (e.g. use a logarithmic scale, modify display of factors, etc.)
• defining facets → create small multiples, each showing a different subset of the data
• changing the overall appearance of the plot by adjusting its theme (e.g. change background color, rotate axis labels, etc.)

You combine all these steps with a +.

We usually work with two-dimensional vector data: Points located within a coordinate reference system (CRS).

• the point -73.94°, 40.6° is represented in the WGS84 CRS: most popular world coord. system (e.g. used by GPS)
• this CRS is ellipsoidal; its components are given in degrees:
  • -73.94° is the longitude: about 74° West from the meridian
  • 40.6° is the latitude: about 41° North from the equator

Let's put this point into context:

• points can be connected to form other geometrical shapes such as lines or polygons
• points and shapes can represent numerous geographic entities: cities, buildings, countries, rivers, lakes, etc.

Most packages for working with geo-data in R rely on the Simple Features standard (most prominently the sf package).

Simple features describe how objects in the real world can be represented in computers in terms of their spatial geometry:

Features have a geometry describing where on Earth the feature is located, and they have attributes, which describe other properties. – sf package vignette

Examples:

• geometry: point at -73.94°, 40.6°
• name: New York City
• population: 8.6 mio.

• geometry: polygons consisting of points at …
• name: Italy
• most popular meal: pizza

First, we load the packages that we need:

library(ggplot2)
library(maps)
library(sf)

The function map can be used to load the "world" data. We need to convert it to a "simple features" object via st_as_sf():

worldmap_data <- st_as_sf(map('world', plot = FALSE, fill = TRUE))
head(worldmap_data, n = 3)  # to have a look inside

## Simple feature collection with 3 features and 1 field
## geometry type:  MULTIPOLYGON
## dimension:      XY
## bbox:           xmin: -8.68335 ymin: 18.98662 xmax: 74.89131 ymax: 42.64795
## epsg (SRID):    4326
## proj4string:    +proj=longlat +datum=WGS84 +no_defs
##                         geometry          ID
## 1 MULTIPOLYGON (((74.89131 37... Afghanistan
## 2 MULTIPOLYGON (((20.06396 42...     Albania
## 3 MULTIPOLYGON (((8.576563 36...     Algeria

• dataset with a "header" giving some general information
• two columns: geometry (spatial data) and ID ("meta data")

What do the numbers in the geometry column mean? What's their CRS?

We have some cities along with their longitude (lng) and latitude (lat):

##       name    lng    lat
## 1   Berlin  13.38  52.52
## 2 New York -73.94  40.60
## 3   Sydney 151.21 -33.87

We add a "point geom" layer to our map:

ggplot(some_cities) +                                # pass the cities data to plot
    geom_sf(data = worldmap_data) +                  # layer 1: world countries
    geom_point(aes(x = lng, y = lat), color = 'red') # layer 2: points at city coord.

You may also add text labels for the cities. ggplot2 provides geom_text and geom_label (draws box around text):

ggplot(some_cities) +                                  # pass the cities data to plot
    geom_sf(data = worldmap_data) +                    # layer 1: world countries
    geom_point(aes(x = lng, y = lat), color = 'red') + # layer 2: points
    geom_label(aes(x = lng, y = lat, label = name),    # layer 3: labels
               hjust = 0, vjust = 1, nudge_x = 3) +    # labels appear next to point
    theme(axis.title = element_blank(),
          axis.text = element_blank())                 # disable the axis labels

Suppose you have observations that refer to geographic locations, e.g. countries, cities, neighborhoods. You may want to:

• link that data with existing information on that geographic entity e.g. country GDP, city population, poverty rate in the neighboorhood
• visualize that data geographically

For both things you need to:

1. agree on geographic (administrative) level for matching (e.g. country, state, municipality)
2. find identifiers to match the observations w/ respective geographic entities

TODO: schematische darstellung

• different names may refer to the same country (e.g. Germany / Federal Republic of Germany, BRD, etc.) → often not a good identifier
• ISO 3166-1 designates every country a two- or three-letter code (e.g. DE / DEU)
• often used in datasets (e.g. from the UN)

The Nomenclature of Territorial Units for Statistics (NUTS) divides the EU territory into regions at 3 different levels for socio-economic analyses of the regions.

Candidate countries and countries inside EFTA also have NUTS regions (Norway, Switzerland, Albania, Turkey, etc.).

In Germany, regions are hierarchically structured and identified by "Amtlicher Gemeindeschlüssel" (AGS ~ "municipality identificator"). The Gemeindeverzeichnis contains all regions with their name, AGS, area, population and other features.

Since 2006, Berlin uses "Lebensweltlich orientierte Räume" (LOR) to structure the city area into sub-regions at different levels, each of them identified by "LOR" code.

1. Prognoseräume (60 regions)
2. Bezirksregionen (138 regions)
3. Planungsräume (447 regions)

There are numerous file formats that store data for geographic information system (GIS) software. The most popular include:

• ESRI shapefile ("SHP"):
  • consists of at least three files in the same directory with filename extensions .shp, .shx and .dbf
• GeoJSON (.geojson, .json) and TopoJSON (.topojson, .json)
• KML (.kml, .kmz): Keyhole Markup Language, used by Google Earth
• GML (.gml): Geography Markup Language
• AutoCAD DXF (.dxf)

You may also encounter other formats such as vector data files (.svg, .poly) or databases (.sql, .sqlite).

Good news: sf can handle all popular file formats.

Two datasets:

• to combine these datasets, the "city" column might be used as identifier for matching
• not all cities in "pm" also appear in "city_coords" and vice versa

Datasets can also be joined with merge, but I find dplyr easier to use.

left_join(a, b, by = <criterion>): always retains rows on the "left side" and fills up non-matching rows with NAs.

library(dplyr)
left_join(pm, city_coords, by = 'city')

##        city pm_mg      lng      lat
## 1     Amman   999 31.91034 35.94846
## 2  Saltillo   869  1.00000  1.00000
## 3      Usak   814  1.00000  1.00000
## 4 The Bronx   284       NA       NA
## 5     Seoul   129  1.00000  1.00000

right_join(a, b, by = <criterion>): always retains rows on the "right side" and fills up non-matching rows with NAs. How many rows do you expect for a right join between pm and city_coords? How many of them will contain an NA value?

right_join(pm, city_coords, by = 'city')

##       city pm_mg      lng      lat
## 1    Amman   999 31.91034 35.94846
## 2 Saltillo   869  1.00000  1.00000
## 3   Berlin    NA  1.00000  1.00000
## 4     Usak   814  1.00000  1.00000
## 5 New York    NA  1.00000  1.00000
## 6    Seoul   129  1.00000  1.00000

inner_join(a, b, by = <criterion>): only retains rows that match on both sides.

How many rows do you expect for an inner join between pm and city_coords?

inner_join(pm, city_coords, by = 'city')

##       city pm_mg      lng      lat
## 1    Amman   999 31.91034 35.94846
## 2 Saltillo   869  1.00000  1.00000
## 3     Usak   814  1.00000  1.00000
## 4    Seoul   129  1.00000  1.00000